Bimetallic and trimetallic oxides and phosphates: Preparation of nanocomposites and their electrochemistry
Lee, Chia Ying
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The motivation for this research is to study two types of inorganic compounds including metal oxides and metal phosphates and further develop new cathode fabrication approaches for potential battery applications. This work covers four prospects of studies: material design and synthesis, physical and chemical property characterizations, cathode fabrication developments, and battery performance evaluations. Sodium Vanadium oxide (Na x V 2 O 5 *nH 2 O) with x= 0.12, 0.17, 0.25, 0.32 were successfully prepared by a novel sol-gel synthesis strategy. These series of amorphous and nanocrystalline materials were characterized with different hydration levels (n= 0.15, 1.08, 1.18, 1.28) correspondingly. Samples with lower sodium content had higher water content. The crystal structure was measured by x-ray powder diffraction (XRD). The interlayer spacing is directly proportional to the water level and inversely proportional to the sodium level. Electrochemical assessment of the sodium vanadium oxide samples was undertaken using experimental cells and lithium anodes. The difference in delivered capacity as related to the sodium and water content was consistent at all three discharge rates tested (C/20, C/5 and C/2) showing a strong linear relationship of increasing capacity with decreasing water content in each case. These data highlight the significant impact of interlayer water content on delivered capacity with a strong linear relationship of increasing capacity with decreasing water content. The capacity differences were maintained as the cells were repeatedly cycled. Two cathode fabrication methods were demonstrated by using Na x V 2 O 5 gels as active materials and carbon nanotubes (CNTs) as conductive substrates. The first method using the material deposited on the CNT substrate after isolation and the second method using direct integration during material synthesis. The second method enabled active material penetration and coating of the CNT-S such that no additional binder or conductive material was needed to achieve viable discharge performance. The metal oxide/CNT-S cathode composites were tested in experimental rechargeable cells. Calculations show that the use of CNT-S can increase the cathode specific capacity by 20–60% by eliminating foil current collector, binders and other inert conductive carbons. The elimination of metal foil on the cathode of a battery may enhance long term stability of the cathode structure and enable use of electrolytes that are currently not viable due to grid corrosion. Magnesium vanadium oxide (MgV2O6) gel is first successfully prepared via sol-gel synthesis approach. Characterizations in Powder X-ray diffraction (XRD), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Differential Scanning Calorimetry (DSC), and Scanning Electron Microscope (SEM) are performed. Study on galvanostat, and cyclic voltammetry indicates the potential application in rechargeable battery systems. Silver vanadyl diphosphate (Ag 2 VP 2 O8) was prepared by the solid state synthesis approach and characterized with physical and chemical property analysis. A notable feature of Ag 2 VP 2 O 8 cathodes is their high thermal stability which bodes well for safety of batteries. We report here the first study of the electrochemical reduction of a silver vanadium diphosphate, Ag 2 VP 2 O 8 , including characterization of the reduced material. In-situ silver formation was observed with Scanning Electron Microscope (SEM) and the investigation on the battery resistances is performed. The correlated electrochemical performance with material composition and thermal stability were demonstrated. Characterization and electrochemical studies on micronized Ag 2 VP 2 O 8 were reported. Decrease in particle sizes and increase in surface areas were demonstrated after micronization without alternating the material nature. Improvement on cell galvanic capacity and pulse capability were also observed. Further investigation on the correlation between the physical properties and electrochemical performance were illustrated by the SEM images of discharged Ag 2 VP 2 O 8 . The combined effects from micronization and in-situ silver formation contribute to the electrochemical performances. Additional discovery on recyclability were achieved by partial discharge strategy. Synthesis and characterization of alluaudite-like compound, Ag 2 FeMn 2 (PO 4 ) 3 ), is reported. Powder X-ray diffraction (XRD), Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Differential Scanning Calorimetry (DSC), Laser Diffraction Particle Size Analyzer (LPS), and Scanning Electron Microscope (SEM) are performed. Investigation on electrochemical performance indicates the potential of rechargeable battery application along with the in-situ silver formation was observed.